Traffic volumes of backbone networks have rapidly increased with the spread of the Internet. In order to cope with this trend, speeding up in long-distance optical communication is required strongly. Techniques coping with the speeding up of optical communication include an optical phase modulation system utilizing a digital signal processing technique and a polarization multiplex separation technique. A technique in which the optical phase modulation system and the polarization multiplex separation technique are combined, that is, a so-called optical digital coherent communication system can realize speeding up in long-distance optical communication, and thus has attracted attention in recent years.
Meanwhile, Patent Document 1 discloses a technique in which, in a Mach-Zehnder optical modulator, the power density of a carrier spectral component is minimized. In addition, Patent Document 1 also discloses a technique in which the power density of a sideband wave is maximized instead of the power density of the carrier spectral component being minimized.